During testing of explosive casks dropped from balloons, one misfired. Someone had punched a hole in the top of the cask and in the bag of gunpowder. When the wick was lit and dropped, it started spewing sparks. It spewed them so hard out of the small hole in the top that it propelled the cask in a spiral flight. Then it finally exploded in flight.
Thomas was watching the test from the ground and saw what happened. His mind began racing with the possibilities it presented. He went to the forge building and began designing a paper tube that would function like the cask. He added a small bag of gunpowder inside it, with a small hole in the bag. He ran a wick through a circle of thin paper and plugged the end of the wick into the loose powder. He pushed the paper circle in the back of the tube. He took this outside the city and put it on a small stand. He lit it. It sprayed sparks and flew in a wild spiral, far up in the air and out of control. Gendry had come with him to see what he was up to, and after watching it fly, he said, "It needs fletches like an arrow." Thomas laughed at him and nodded.
They tried one with actual arrow fletches. It was better, but still out of control in flight. They made bigger fletches from hard paper, and it flew like an arrow, a nice, tight spiral, and a very long flight path. Gendry punched him in the side, laughing as they watched him fly. Weeks went by as they refined this thing until they could predict its flight path and distance, until they had determined its usefulness.
They purchased a derelict fishing boat and had it anchored off the coast. They devised a way to add an explosive charge to the end of the tube's flight path, so it would detonate when it hit. It took them three days of testing to discover the exact amount of gunpowder to add to the tube to hit the anchored vessel. The younger Mangana was with them as they did this testing and said, "Could you make the tube from light, thin metal?"
Gendry had some sheets of thin tin, so he cut out six tubes' worth, along with the fletches to go with them. The first one they tested just blew out all its sparks and didn't move. So they added a nozzle to the end. When they spat fire and were gone so fast, it shocked the three of them. It overshot the fishing vessel by 100 feet. Now they were getting somewhere. With more tinkering and testing over the next weeks, they refined their device into a ship-sinking machine. By raising and lowering the angle of their launch platform, they could make it go even further.
This was a weapon that might swing the battle into their favor. They finalized all the plans and had them sent to Bran. Through a telepathic link, Bran said he loved it and that they would begin mass-producing them.
Fall turned into winter again, and the large buildings to house the balloons were finished in time for the first snows. Thomas and Arya were spending time with the twins, as they weren't just little children anymore; they were young adults, almost. The two proud parents grinned at each other as they watched their babies become little people before their eyes. And their resolve to win the upcoming battles became even stronger to protect their children's lives.
Fire arrow batteries were set up along the coast, and the ships were equipped with them as well. Brandon's Rest now had eleven ships to throw into the fight. King's Landing had twenty. Seventeen Sea Lord ships were also given to add to the fight. These didn't have wind-power affinity users, but they did have guns mounted on them. Forty-eight ships to throw at 1000 enemy ships. But they had a plan, and they had guns.
The winter grew long, and it snowed heavily at Brandon's Rest, but they were all warm by their fires, and they tried to spend as much time around family and friends as they could. They drank, and they ate as if there wasn't a life-threatening force waiting for spring to end all their lives.